It might be a cliche, but sometimes, a picture says more than a thousand words. Over the years, I've often talked about how the technology world is iterative, about how products are virtually always built upon that which came before, about how almost always, multiple people independently arrive at the same products since they work within the same constraints of the current state of technology. This elementary aspect of the technology world, which some would rather forget, has been illustrated very, very well in one of Samsung's legal filings against Apple.

Why wouldn't it be considered as such? (especially if we're include Simon or Treo)

Anyway, there's perhaps one even earlier device that Samsung could point out - perhaps a bit on the large side of things, but still showing similar form-factor: GRiDPad, a tablet launched in 1989 http://www.computinghistory.org.uk/det/6565/GRidPad-1910/
And the best part: it was manufactured by... Samsung.